Particle morphology can be characterized, in general, by three properties: sphericity, roundness, and roughness. These properties are sometimes referred to by other names such as shape, angularity, and surface roughness, respectively. In either case, these properties are scale dependent, as they measure morphological characteristics at different length scales, with increased spatial resolution needed to measure roughness, for example. Particle morphology has been shown to be crucially important for macroscopic properties in granular materials. Some of the most critical macroscopic properties used in engineering and science are strength and permeability, and both of these are intimately affected by particle morphology. In the case of macroscopic strength, it has been determined that lack of sphericity, sharper angularity and increased roughness all lead to increased mobilized strength in granular materials. This macroscopic effect is due to micromechanical effects such as increased number of contact points. Therefore, accurate micromechanical models for granular materials should be able to correctly capture particle morphology if they are to correctly predict the macroscopic strength in real materials such as sands.
The Discrete Element Method (DEM) was introduced in order to reveal micromechanical features that were simply not accessible to continuum models. This modeling paradigm has allowed tremendous access to understand most features of the micromechanical behavior of granular materials and link them with macroscopic response. DEM starts with the underlying assumption that particles are discrete elements and by utilizing numerical methods computes the motion and effect of a large number of such particles. A DEM simulation typically involves first generating a model that results in spatially orienting all particles and assigning an initial velocity. Next, the forces that act on each individual particle can be calculated using initial data, relevant physical laws, and particle contact models. All the forces are added together to find the total force acting on each particle and an integration method computes the change in the position and the velocity of each particle during a particular time step. The resulting new positions of the particles are used to compute forces for the next time step and the process is repeated until the predetermined time period for the simulation is completed. DEM is a powerful analytical tool and has applications in chemical, civil engineering, oil and gas, mining, mineral processing and pharmaceutical industries to name a few.
Increases in computational power have allowed for longer simulations and the inclusion of more particles when conducting DEM calculations. However, the link to macroscopic response remains mainly qualitative (as opposed to quantitative) in large part due to the challenges within DEM of accurately capturing particle morphology. Clustering and polyhedra are currently two widely discussed techniques that can be used to capture three-dimensional particle morphology in natural materials. Clustering is relatively simple to implement in current codes but cannot represent particle curvature readily, which could impact contact properties such as normal and tangential forces, thereby potentially changing macroscopic response. On the other hand, polyhedra can represent complex geometrical entities fairly well (especially in two dimensions), but contact detection algorithms available for these geometrical entities are rather complex (especially in three dimensions) as they introduce the need to deal with face-to-node, node-to-node, and face-to-face contact.
New characterization tools such as X-ray computed tomography (CT) enable direct measurement of particle morphology, as a function of imaging resolution. Generally, X-ray CT utilizes computer-processed X-rays to produce tomographic images or cross-sectional images of a particle non-destructively. Although, the resulting images provide accurate measurements of particle morphologies, the current DEM techniques such as clustering and polyhedral based methods are limited in their ability to capture particle morphology for the purpose of performing quantitative analysis of macroscopic response.